Rendering depth of field through transparent objects in blender render
This is the downside of post processed depth of field.
You see, the depth map is a 2D image. To create it, rays get shot into the scene and depending on how far away from the camera they hit an object, the corresponding pixel gets a grey value. (white if in front of the camera, black if far away)

Because of that there is no information in the depth map how far away the objects in the background are. The same problem occurs if you render through a glass window. The depth map would be totally flat in that case.

So I would recommend rendering the scene with camera dof turned on, rather than adding it in post. Also, adding dof in post is a cheat and will never look as well.